A Vietnamese dissident known for leading anti-Chinese and environmental protests says he was attacked by a group of unidentified men allegedly linked to the authorities.

Anh Chi told VOA’s Vietnamese Service he was hit Monday with a metal pipe without provocation near his home in Hanoi.

Photos of his bloodied head and face have been widely circulated on activist web sites, but none of the photos have been seen on state-owned media.

Chi believes he has been harassed by local authorities in recent weeks for being outspoken against state officials.

“Social activists like me know for sure who is behind such an attack. Even thugs do not attack someone without a reason. We know who stands behind it," said Chi.

Vietnamese officials could not be reached for comment, and Hanoi has not responded to the allegation.

The incident came two days after Vietnam and the U.S. concluded an annual human rights dialogue described as productive by Tom Malinowski, U.S. Assistant Secretary of State for Democracy, Human Rights and Labor.”

Anh Chi was actively involved in a campaign to hold officials accountable for a plan to fell thousands of trees, but later scrapped after facing widespread public outcry.

On his official Twitter account, Malinowski retweeted a post by Vietnamese American human rights lawyer Vi K Tran that read “#Vietnam gov't can't say it listens to tree cutting protests while beating protesters.”

During his meetings in Hanoi, Malinowski said that while “Vietnam has made progress on human rights” in recent years, “very significant problems” remain.

“Civil society activists, bloggers, independent journalists and other critics of the government still too often face harassment, threats and even violence for exercising what ought to be their internationally recognized human rights," said Malinowski.

Malinowski added that human rights is still “a sensitive and difficult issue in the relationship between the United States and Vietnam.”

Relations between the two former foes have been strengthened in recent years, but the human rights issue has overshadowed gains in other areas.

The U.S. and Vietnam have been in negotiations with other countries to conclude the long-delayed Trans-Pacific Partnership (TPP) trade pact. But worker rights in Vietnam is a thorny issue.

Speaking at Nike headquarters in Oregon last week while seeking support for the trade deal, President Barack Obama said “under this agreement, Vietnam would actually, for the first time, have to raise its labor standards."

“It would have to set a minimum wage. It would have to pass safe workplace laws to protect its workers. It would even have to protect workers’ freedom to form unions for the very first time. That would make a difference. That helps to level the playing field and it would be good for the workers in Vietnam, even as it helps make sure that they’re not undercutting competition here in the United States," said President Obama.

Top Vietnamese officials have expressed strong desire to join the TPP, but it faces resistance in the U.S. Congress, even among members from the president's own party.

Some U.S. lawmakers have stressed that Vietnam should not be granted the membership until it makes significant labor and human rights reforms.

This report was produced in collaboration with the VOA Vietnamese service.